Connecting tv to phone via hdmi cable slows down phone badly

I've recently bought Xperia S, and overall I'm very happy with the phone and one of the reasons I bought this specific model was separate HDMI output.

Unfortunately I seem to have some problems when i connect my phone to tv.

While i get video and audio, the phone seems to slow down dramaticly, the ui starts to get laggy ans way less responsive than it would be normally.

Actions like bringing the notification area or app browser are not fluid anymore.

Also games like "cut the rope", etc have very bad performance. it instantly gets fixed if i unplug hdmi cable out of the phone.

I'm not sure what is the reason, but TV always reports the signal as 1080p (i was expecting the phone would output 720p ?), my phone was bought in Poland and firmware version is 6.0.A.3.62.

The "signal" quality setting doesnt seem to do anything visible for me. i was thinking maybe its because phone does some upscaling? I've tried the setting for screen size 80%-100% in hdmi options - but it doesnt seem to have any visible performance impact on my phone sam as with signal quality setting which doesnt seem to do anything at all.